igneous
Basalt
Basalt — fine-grained mafic volcanic rock, primarily plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectUsually dark gray to black, fine-grained, and dull to weakly lustrous; Mohs hardness about 6. Its crystals are generally too small to see, and it has no consistent cleavage.
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Physical properties
Usually dark gray to black, fine-grained, and dull to weakly lustrous; Mohs hardness about 6. Its crystals are generally too small to see, and it has no consistent cleavage.
Formation & geological history
Forms when iron- and magnesium-rich lava cools quickly at or near Earth’s surface. Basalt occurs from modern eruptions to rocks billions of years old.
Uses & applications
Widely used as crushed aggregate for roads and concrete, dimension stone, and railway ballast; some varieties are polished for decorative use.
Geological facts
Basalt makes up most of the oceanic crust and is common in large lava plateaus and volcanic islands.
Field identification & locations
Identify it by its dense, dark color and fine texture; vesicles may mark trapped gas bubbles. Common at volcanic regions and along ocean floors; a photo alone cannot confirm the specimen’s exact composition.
